Biaxially oriented microporous membrane

ABSTRACT

A microporous membrane is made by a dry-stretch process and has substantially round shaped pores and a ratio of machine direction tensile strength to transverse direction tensile strength in the range of 0.5 to 5.0. The method of making the foregoing microporous membrane includes the steps of: extruding a polymer into a nonporous precursor, and biaxially stretching the nonporous precursor, the biaxial stretching including a machine direction stretching and a transverse direction stretching, the transverse direction including a simultaneous controlled machine direction relax.

RELATED APPLICATION

The instant application claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/775,112 filed Feb. 21, 2006.

FIELD OF THE INVENTION

The invention is directed to a biaxially oriented microporous membraneand the method of its manufacture.

BACKGROUND OF THE INVENTION

Microporous membranes are known, can be made by various processes, andthe process by which the membrane is made has a material impact upon themembrane's physical attributes. See, Kesting, R., Synthetic PolymericMembranes, A structural perspective, Second Edition, John Wiley & Sons,New York, N.Y., (1985). Three commercially viable processes for makingmicroporous membranes include: the dry-stretch process (also known asthe CELGARD process), the wet process, and the particle stretch process.

The dry-stretch process refers to a process where pore formation resultsfrom stretching the nonporous precursor. See, Kesting, Ibid. pages290-297, incorporated herein by reference. The dry-stretch process isdifferent from the wet process and particle stretch process. Generally,in the wet process, also know as the phase inversion process, or theextraction process or the TIPS process (to name a few), the polymericraw material is mixed with a processing oil (sometimes referred to as aplasticizer), this mixture is extruded, and pores are then formed whenthe processing oil is removed (these films may be stretched before orafter the removal of the oil). See, Kesting, Ibid. pages 237-286,incorporated herein by reference. Generally, in the particle stretchprocess, the polymeric raw material is mixed with particulate, thismixture is extruded, and pores are formed during stretching when theinterface between the polymer and the particulate fractures due to thestretching forces. See, U.S. Pat. Nos. 6,057,061 and 6,080,507,incorporated herein by reference.

Moreover, the membranes arising from these processes are physicallydifferent and the process by which each is made distinguishes onemembrane from the other. Dry-stretch membranes have slit shaped poresdue to the inability to stretch the precursor in the transverse machinedirection. Wet process membranes have rounder pores due to the abilityto stretch the precursor in the transverse machine direction. Particlestretched membranes, on the other hand, are filled with particulateneeded for pore formation. Accordingly, each membrane may bedistinguished from the other by its method of manufacture.

While membranes made by the dry-stretch process have met with excellentcommercial success, there is a need to improve their physicalattributes, so that they may be used in wider spectrum of applications.Some areas of improvement include pore shapes other than slits andincrease transverse direction tensile strength.

U.S. Pat. No. 6,602,593 is directed to a microporous membrane, made by adry-stretch process, where the resulting membrane has a ratio oftransverse direction tensile strength to machine direction tensilestrength of 0.12 to 1.2. Herein, the TD/MD tensile ratio is obtained bya blow-up ratio of at least 1.5 as the precursor is extruded.

SUMMARY OF THE INVENTION

A microporous membrane is made by a dry-stretch process and hassubstantially round shaped pores and a ratio of machine directiontensile strength to transverse direction tensile strength in the rangeof 0.5 to 5.0.

The method of making the foregoing microporous membrane includes thesteps of: extruding a polymer into a nonporous precursor, and biaxiallystretching the nonporous precursor, the biaxial stretching including amachine direction stretching and a transverse direction stretching, thetransverse direction including a simultaneous controlled (restrained)machine direction relax.

DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form that is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a photograph of one embodiment of the instant invention(single ply membrane).

FIG. 2 is a photograph of another embodiment of the instant invention(multi-ply membrane, plies laminated together then stretched).

FIG. 3 is a photograph of another embodiment of the instant invention(multi-ply membrane, plies coextruded then stretched).

FIG. 4 is a photograph of a prior art dry-stretched membrane (single plymembrane).

FIG. 5 is a photograph of a prior art dry-stretched membrane (multi-plymembrane, plies laminated then stretched).

DESCRIPTION OF THE INVENTION

A microporous membrane is made by a dry-stretch process and hassubstantially round shaped pores and a ratio of machine directiontensile strength to transverse direction tensile strength in the rangeof 0.5 to 4.0. A microporous membrane is a thin, pliable, polymericsheet, foil, or film having a plurality of pores therethrough. Suchmembranes by be used in a wide variety of applications, including, butnot limited to, mass transfer membranes, pressure regulators, filtrationmembranes, medical devices, separators for electrochemical storagedevices, membranes for use in fuel cells, and the like.

The instant membrane is made by the dry-stretch process (also known asthe CELGARD process). The dry-stretch process refers to a process wherepore formation results from stretching the nonporous precursor. See,Kesting, R., Synthetic Polymeric Membranes, A structural perspective,Second Edition, John Wiley & Sons, New York, N.Y., (1985), pages290-297, incorporated herein by reference. The dry-stretch process isdistinguished from the wet process and particle stretch process, asdiscussed above.

The instant membrane may be distinguished from prior dry-stretchedmembranes in at least two ways: 1) substantially round shape pores, and2) a ratio of machine direction tensile strength to transverse directiontensile strength in the range of 0.5 to 4.0.

Regarding the pore shape, the pores are characterized as substantiallyround shaped. See, FIGS. 1-3. This pore shape is contrasted with theslit shaped pores of the prior art dry-stretched membranes. See FIGS.4-5 and Kesting, Ibid. Further, the pore shape of the instant membranemay be characterized by an aspect ratio, the ratio of the length to thewidth of the pore. In one embodiment of the instant membrane, the aspectratio ranges from 0.75 to 1.25. This is contrasted with the aspect ratioof the prior dry-stretched membranes which are greater than 5.0. SeeTable below.

Regarding the ratio of machine direction tensile strength to transversedirection tensile strength, in one embodiment, this ratio is between 0.5to 5.0. This ratio is contrasted with the corresponding ratio of theprior art membranes which is greater than 10.0. See Table below.

The instant membrane may be further characterized as follows: an averagepore size in the range of 0.03 to 0.30 microns (μ); a porosity in therange of 20-80%; and/or a transverse direction tensile strength ofgreater than 250 Kg/cm². The foregoing values are exemplary values andare not intended to be limiting, and accordingly should be viewed asmerely representative of the instant membrane.

The polymers used in the instant membrane may be characterized asthermoplastic polymers. These polymers may be further characterized assemi-crystalline polymers. In one embodiment, semi-crystalline polymermay be a polymer having a crystallinity in the range of 20 to 80%. Suchpolymers may be selected from the following group: polyolefins,fluorocarbons, polyamides, polyesters, polyacetals (orpolyoxymethylenes), polysulfides, polyvinyl alcohols, co-polymersthereof, and combinations thereof. Polyolefins may include polyethylenes(LDPE, LLDPE, HDPE, UHMWPE), polypropylene, polybutene,polymethylpentene, co-polymers thereof, and blends thereof.Fluorocarbons may include polytetrafluoroethylene (PTFE),polychlorotrifluoroethylene (PCTFE), fluorinated ethylene propylene(FEP), ethylene chlortrifluoroethylene (ECTFE), ethylenetetrafluoroethylene (ETFE), polyvinylidene fluoride (PVDF),polyvinylfluoride (PVF), prefluoroalkoxy (PFA) resin, co-polymersthereof, and blends thereof. Polyamides may include, but are not limitedto: polyamide 6, polyamide 6/6, Nylon 10/10, polyphthalamide (PPA),co-polymers thereof, and blends thereof. Polyesters may includepolyester terephthalate (PET), polybutylene terephthalate (PBT),poly-1-4-cyclohexylenedimethylene terephthalate (PCT), polyethylenenaphthalate (PEN), and liquid crystal polymers (LCP). Polysulfidesinclude, but are not limited to, polyphenylsulfide, polyethylenesulfide, co-polymers thereof, and blends thereof. Polyvinyl alcoholsinclude, but are not limited to, ethylene-vinyl alcohol, co-polymersthereof, and blends thereof.

The instant membrane may include other ingredients, as is well known.For example, those ingredients may include: fillers (inert particulatesused to reduce the cost of the membrane, but otherwise having nosignificant impact on the manufacture of the membrane or its physicalproperties), anti-static agents, anti-blocking agents, anti-oxidants,lubricants (to facilitate manufacture), and the like.

Various materials may be added to the polymers to modify or enhance theproperties of the membrane. Such materials include, but are not limitedto: (1) polyolefins or polyolefin oligomers with a melting temperatureless than 130° C.; (2) Mineral fillers include, but are not limited to:calcium carbonate, zinc oxide, diatomaceous earth, talc, kaolin,synthetic silica, mica, clay, boron nitride, silicon dioxide, titaniumdioxide, barium sulfate, aluminum hydroxide, magnesium hydroxide and thelike, and blends thereof; (3) Elastomers include, but are not limitedto: ethylene-propylene (EPR), ethylene-propylene-diene (EPDM),styrene-butadiene (SBR), styrene isoprene (SIR), ethylidene norbornene(ENB), epoxy, and polyurethane and blends thereof; (4) Wetting agentsinclude, but are not limited to, ethoxylated alcohols, primary polymericcarboxylic acids, glycols (e.g., polypropylene glycol and polyethyleneglycols), functionalized polyolefins etc; (5) Lubricants, for example,silicone, fluoropolymers, Kemamide®, oleamide, stearamide, erucamide,calcium stearate, or other metallic stearate; (6) flame retardants forexample, brominated flame retardants, ammonium phosphate, ammoniumhydroxide, alumina trihydrate, and phosphate ester; (7) cross-linking orcoupling agents; (8) polymer processing aid; and (9) Any types ofnucleating agents including beta-nucleating agent for polypropylene.(The instant membrane, however, specifically excludes any beta-nucleatedpolypropylene as disclosed in U.S. Pat. No. 6,602,593, incorporatedherein by reference. A beta-nucleated polypropylene is a substance thatcauses the creation of beta crystals in polypropylene.)

The instant membrane may be a single ply or multi-ply membrane.Regarding the multi-ply membrane, the instant membrane may be one ply ofthe multi-ply membrane or the instant membrane may be all of the pliesof the multi-ply membrane. If the instant membrane is less than all ofthe plies of the multi-ply membrane, the multi-ply membrane may be madevia a lamination process. If the instant membrane is all plies of themulti-ply membrane, the multi-ply membrane may be made via an extrusionprocess. Further, multi-ply membranes may be made of plies of the samematerials or of differing materials.

The instant membrane is made by a dry-stretch process where theprecursor membrane is biaxially stretched (i.e., not only stretched inthe machine direction, but also in the transverse machine direction).This process will be discussed in greater detail below.

In general, the process for making the foregoing membrane includes thesteps of extruding a nonporous precursor, and then biaxially stretchingthe nonporous precursor. Optionally, the nonporous precursor may beannealed prior to stretching.

In one embodiment, the biaxial stretching includes a machine directionstretch and a transverse direction with a simultaneous controlled(restrained) machine direction relax.

The machine direction stretch and the transverse direction stretch maybe simultaneous or sequential. In one embodiment, the machine directionstretch is followed by the transverse direction stretch with thesimultaneous machine direction relax. This process is discussed ingreater detail below.

Extrusion is generally conventional (conventional refers to conventionalfor a dry-stretch process). The extruder may have a slot die (for flatprecursor) or an annular die (for parison precursor). In the case of thelatter, an inflated parison technique may be employed (e.g., a blow upratio (BUR)). However, the birefringence of the nonporous precursor doesnot have to be as high as in the conventional dry-stretch process. Forexample, in the conventional dry-stretch process to produce a membranewith a >35% porosity from a polypropylene resin, the birefringence ofthe precursor would be >0.0130; while with the instant process, thebirefringence of the PP precursor could be as low as 0.0100. In anotherexample, a membrane with a >35% porosity from a polyethylene resin, thebirefringence of the precursor would be >0.0280; while with the instantprocess, the birefringence of the PE precursor could be as low as0.0240.

Annealing (optional) may be carried out, in one embodiment, attemperatures between T_(m)−80° C. and T_(m)−10° C. (where T_(m) is themelt temperature of the polymer); and in another embodiment, attemperatures between T_(m)−50° C. and T_(m)−15° C. Some materials, e.g.,those with high crystallinity after extrusion, such as polybutene, mayrequire no annealing.

Machine direction stretch may be conducted as a cold stretch or a hotstretch or both, and as a single step or multiple steps. In oneembodiment, cold stretching may be carried out at <T_(m)−50° C., and inanother embodiment, at <T_(m)−80° C. In one embodiment, hot stretchingmay be carried out at <T_(m)−10° C. In one embodiment, total machinedirection stretching may be in the range of 50-500%, and in anotherembodiment, in the range of 100-300%. During machine direction stretch,the precursor may shrink in the transverse direction (conventional).

Transverse direction stretching includes a simultaneous controlled(restrained) machine direction relax.

This means that as the precursor is stretched in the transversedirection the precursor is simultaneously allowed to contract (i.e.,relax), in a controlled (restrained) manner, in the machine direction.

The transverse direction stretching may be conducted as a cold step, ora hot step, or a combination of both. In one embodiment, totaltransverse direction stretching may be in the range of 100-1200%, and inanother embodiment, in the range of 200-900%. In one embodiment, thecontrolled machine direction relax may range from 5-80%, and in anotherembodiment, in the range of 15-65%. In one embodiment, transversestretching may be carried out in multiple steps. During transversedirection stretching, the precursor may or may not be allowed to shrinkin the machine direction. In an embodiment of a multi-step transversedirection stretching, the first transverse direction step may include atransverse stretch with the controlled machine relax, followed bysimultaneous transverse and machine direction stretching, and followedby transverse direction relax and no machine direction stretch or relax.

Optionally, the precursor, after machine direction and transversedirection stretching may be subjected to a heat setting, as is wellknown.

The foregoing membrane and process are further illustrated in thefollowing non-limiting examples.

EXAMPLES

The test values reported herein, thickness, porosity, tensile strength,and aspect ratio, were determined as follows: thickness—ASTM-D374 usingthe Emveco Microgage 210-A micrometer; porosity—ASTM D-2873; tensilestrength—ASTM D-882 using an Instron Model 4201; and aspectratio-measurements taken from the SEM micrographs.

The following examples were produced by conventional dry-stretchedtechniques, except as noted.

Example 1

Polypropylene (PP) resin is extruded using a 2.5 inch extruder. Theextruder melt temperature is 221° C. Polymer melt is fed to a circulardie. The die temperature is set at 220° C., polymer melt is cooled byblowing air. Extruded precursor has a thickness of 27μ and abirefringence of 0.0120. The extruded film was then annealed at 150° C.for 2 minutes. The annealed film is then cold stretched to 20% at roomtemperature, and then hot stretched to 228% and relaxed to 32% at 140°C. The machine direction (MD) stretched film has a thickness of 16.4micron (μ), and porosity of 25%. The MD stretched film is thentransverse direction (TD) stretched 300% at 140° C. with MD relax of50%. The finished film has a thickness of 14.1 microns, and porosity of37%. TD tensile strength of finished film is 550 Kg/cm². See FIG. 1.

Example 2

Polypropylene (PP) resin is extruded using a 2.5 inch extruder. Theextruder melt temperature is 220° C. Polymer melt is fed to a circulardie. The die temperature is set at 200° C., polymer melt is cooled byblowing air. Extruded precursor has a thickness of 9.5μ and abirefringence of 0.0160. HDPE resin is extruded using a 2.5 inchextruder. The extruder melt temperature is 210° C. Polymer melt is fedto a circular die. Die temperature is set at 205° C., polymer melt iscooled by air. Extruded precursor has a thickness of 9.5μ and abirefringence of 0.0330. Two PP layers and one PE layers are laminatedtogether to form a PP/PE/PP tri-layer film. Lamination roll temperatureis 150° C. Laminated tri-layer film is then annealed at 125° C. for 2minutes. The annealed film is then cold stretched to 20% at roomtemperature, and then hot stretched to 160% and relaxed to 35% at 113°C. The MD stretched film has a thickness of 25.4 micron, and porosity of39%. The MD stretched film is then TD stretched 400% at 115° C. with MDrelax of 30%. The finished film has a thickness of 19.4 microns andporosity of 63%. TD tensile strength of finished film is 350 Kg/cm². SeeFIG. 2.

Example 3

PP resin and HDPE resin are extruded using a co-extrusion die to form aPP/PE/PP tri-layer film. Extruder melt temperature for PP is 243° C.,and extruder melt temperature for PE is 214° C. Polymer melt is then fedto a co-extrusion die which is set at 198° C. Polymer melt is cooled byblowing air. The extruded film has a thickness of 35.6 microns. Theextruded precursor is then annealed at 125° C. for 2 minutes. Theannealed film is then cold stretched to 45% at room temperature and hotstretched to 247% and relaxed to 42% at 113° C. The MD stretched filmhas a thickness of 21.5 microns and porosity of 29%. The MD stretchedfilm is then TD stretched 450% at 115° C. with 50% MD relax. Thefinished film has a thickness of 16.3 microns and porosity of 59%. TDtensile strength of finished film is 570 Kg/cm².

Example 4

PP resin and HDPE resin are co-extruded and MD stretched the same way asin example 3. The MD stretched film is then TD stretched 800% at 115° C.with 65% MD relax. The finished film has a thickness of 17.2 microns andporosity of 49%. TD tensile strength of finished film is 730 Kg/cm². SeeFIG. 3.

Example 5

PP resin and PB resin are extruded using a co-extrusion die. Extrudermelt temperature for PP is 230° C., and extruder melt for PB is 206° C.Polymer melt is then fed to a co-extrusion die which is set at 210° C.Polymer melt is then cooled by blowing air. The extruded film has athickness of 36.0 microns. The extruded precursor is then annealed at105° C. for 2 minutes. The annealed film is then cold stretched to 20%,and then hot stretched at 105° C. to 155% and then relaxed to 35%. TheMD stretched film is then TD stretched 140% at 110° C. with 20% MDrelax. The finished film has a thickness of 14.8 microns and porosity of42%. TD tensile strength of finished film is 286 Kg/cm².

Example 6

PP resin and PE resin are extruded using a co-extrusion die to form aPP/PE/PP trilayer film. Extruder melt temperature for PP is 245° C., andextruder melt temperature for PE is 230° C. Polymer melt is then fed toa co-extrusion die which is set at 225° C. Polymer melt is cooled byblowing air. The extruded film has a thickness of 27 microns and abirefringence of 0.0120. The extruded precursor is then annealed at 115°C. for 2 minutes. The annealed film is then cold stretched to 22% atroom temperature and hot stretched to 254% and relaxed to 25% at 120° C.(total machine direction stretch=251%). The MD stretched film has athickness of 15 microns and porosity of 16%. The MD stretched film isthen TD stretched 260% at 130° C. with 50% MD relax, followed by asimultaneous MD and TD stretch of 50% and 216% in each direction at 130°C., and finally the film is held fast in the MD (100%) and allowed torelax 57.6% in the TD at a temperature of 130° C. The finished film hasa thickness of 7.6 microns and porosity of 52%. TD tensile strength offinished film is 513 Kg/cm².

Example 7

PP resin and PE resin are extruded using a co-extrusion die to form aPP/PE/PP trilayer film. Extruder melt temperature for PP is 222° C., andextruder melt temperature for PE is 225° C. Polymer melt is then fed toa co-extrusion die which is set at 215° C. Polymer melt is cooled byblowing air. The extruded film has a thickness of 40 microns andbirefringence of 0.0110. The extruded precursor is then annealed at 105°C. for 2 minutes. The annealed film is then cold stretched to 36% atroom temperature and hot stretched to 264% and relaxed to 29% at 109° C.(total machine direction stretch=271%). The MD stretched film has athickness of 23.8 microns and porosity of 29.6%. The MD stretched filmis then TD stretched 1034% at 110° C. with 75% MD relax. The finishedfilm has a thickness of 16.8 microns and porosity of 46%. TD tensilestrength of finished film is 1037 Kg/cm².

In the following table the results of the foregoing experiments aresummarized and compared to two commercially available dry-stretchedmembranes: A) CELGARD® 2400 (single ply polypropylene membrane), SeeFIG. 4; and B) CELGARD® 2300 (tri-layerpolypropylene/polyethylene/polypropylene), see FIG. 5.

TABLE MD TD Tensile Tensile TD Thickness strength strength MD/TD Aspectstretching (um) Porosity (kg/cm²) (kg/cm²) tensile ratio ratio A N/A25.4 37% 160 1700 10.6 6.10 B N/A 25.1 40% 146 1925 13.2 5.50 Ex 1 300%14.1 37% 550 1013 1.8 0.90 Ex 2 400% 19.4 63% 350 627 1.8 0.71 Ex 3 450%16.3 59% 570 754 1.3 — Ex 4 800% 17.2 49% 730 646 0.9 0.83 Ex 5 140%14.8 42% 286 1080 3.8 — Ex 6 418% 7.6 52% 513 1437 2.8 — Ex 7 1034% 16.846% 1037 618 0.6 —

The present invention may be embodied in other forms without departingfrom the spirit and the essential attributes thereof, and, accordingly,reference should be made to the appended claims, rather than to theforegoing specification, as indicated the scope of the invention.Further, all numerical ranges set forth herein should be considered asapproximate ranges and not necessarily as absolute ranges.

We claim:
 1. A method of making a microporous membrane comprising thesteps of: extruding a semi-crystalline polymer into a nonporousprecursor, and biaxially stretching the nonporous precursor, the biaxialstretching including a machine direction stretching and a transversedirection stretching, the transverse direction stretching including asimultaneous restrained machine direction relax in the range of 5 to80%, to form substantially round shaped pores having an aspect ratio inthe range of 0.75 to 1.25, wherein the method is a dry stretch processand the polymer excludes any oils for subsequent removal to form poresor any pore-forming particulate to facilitate pore formation.
 2. Themethod of claim 1 wherein the polymer being selected from the groupconsisting of polyolefins, fluorocarbons, polyamides, polyesters,polyacetals (or polyoxymethylenes), polysulfides, polyvinyl alcohols,co-polymers thereof, and combinations thereof.
 3. The method of claim 1further comprising the step of: annealing the non-porous precursor afterextruding and before biaxially stretching.
 4. The method of claim 3wherein annealing being conducted at a temperature in the range ofT_(m)−80° C. to T_(m)−10° C.
 5. The method of claim 1 wherein biaxiallystretching comprising the steps of: machine direction stretching, andthereafter transverse direction stretching including a simultaneousrestrained machine direction relax.
 6. The method of claim 5 whereinmachine direction stretching being conducted either hot or cold or both.7. The method of claim 6 wherein cold machine direction stretching beingconducted at a temperature <T_(m)−50° C.
 8. The method of claim 6wherein hot machine direction stretching being conducted at atemperature <T_(m)−10° C.
 9. The method of claim 1 wherein the totalmachine direction stretch being in the range of 50-500%.
 10. The methodof claim 1 wherein the total transverse direction stretch being in therange of 100-1200%.
 11. The method of claim 1 wherein the machinedirection relax being in the range of 15 to 65%.
 12. The method of claim1 wherein the machine direction relax being in the range of 50-75%. 13.The method of claim 1 wherein the polymer being one or more polyolefins.14. The method of claim 1 wherein the microporous membrane having aratio of machine direction tensile strength to transverse directiontensile strength in the range of 0.5 to 5.0.
 15. A method of making amicroporous membrane comprising the steps of: extruding asemi-crystalline polymer into a nonporous precursor, and thesemi-crystalline polymer being one or more polyolefins, and biaxiallystretching the nonporous precursor, the biaxial stretching including amachine direction stretching and a transverse direction stretching, thetransverse direction stretching including a simultaneous restrainedmachine direction relax in the range of 5 to 80%, to form substantiallyround shaped pores having an aspect ratio in the range of 0.75 to 1.25,wherein the method is a dry stretch process and the polymer excludes anyoils for subsequent removal to form pores or any pore-formingparticulate to facilitate pore formation.